Memory-bounded streaming builder for /controller. Produces the JSON payload one small unit at a time into a fixed 3KB buffer, so a chunked HTTP response can drain it as the TCP send window allows — no growing cbuf in the async response stream.

src/WResp.h

@@ -52,6 +52,11 @@ class JsonFormatter {
     void addElem(const char *name, const char *val);
     void addElem(const char* name, uint64_t lval);
 };
+class BufferedJsonFormatter : public JsonFormatter {
+  public:
+    void setBuffer(char *b, size_t sz);
+    size_t length() const;
+};
 class AsyncJsonResp : public JsonFormatter {
   protected:
     void _safecat(const char *val, bool escape = false) override;